Direct current (DC) motors are very popular in variable speed drives due to simple speed control and simple control circuits. However, the DC motor initially used hard brushes, due to which the DC motors suffered from a low reliability and required frequent maintenance or replacement. These drawbacks of the DC motors were eliminated by using brushless DC motors (BLDC), which are highly reliable and can be used in applications requiring high speed.
A BLDC motor includes two coaxial magnetic armatures separated by an air gap. An external armature is called a stator and an internal armature is called a rotor. In the BLDC motor, the rotor is a permanent magnet and is supplied by a constant DC current. The stator is poly-phased, three-phases in the present invention, and is coveted by poly-phased currents. Three phase brushless DC motors are used in automotive equipment, refrigerators, air conditioners, compressors and fans due to their high efficiency, silent operation, compact form, reliability and longevity.
FIG. 1 illustrates a circuit diagram of star connected windings for a conventional BLDC motor. The star connected windings of the BLDC motor are connected to commutation switches. The commutation switches can be field effect transistors (FET). The star connected windings, such as coil A, coil B and coil C are connected in a star configuration with a neutral node 4. A node 1 of coil A is connected to switches S1 and S2. A node 2 of coil B is connected to switches S5 and S6 and a node 3 of coil C is connected to switches S3 and S4. The node 4 is unutilized and is kept at an open circuit voltage. The switches S1, S3 and S5 are connected to a supply voltage V and the switches S2, S4 and S6 are connected to a ground voltage. These switches can be controlled by specifically designed devices for motor control applications, like ST7FMC devices, as illustrated in FIG. 2.
Using a single-pole three-phase BLDC motor as illustrated above, one mechanical rotation can be achieved in six steps. Each step corresponds to 60 degrees of rotation, i.e., 360/6 . The six steps are generated by switching different combinations of switches as illustrated in FIG. 3A and FIG. 3B. Step 1 shows a node 1 connected to the positive supply voltage V and the node 3 connected to the ground voltage, by turning the switches S1 and S4 to an on state. A resultant magnetic field will align the rotor in a direction as illustrated in step 1. In Step 2, the switches S1 and S6 are in the on state, so the node 1 is connected to the positive supply voltage V and the node 2 is connected to the ground voltage. The resultant magnetic field will turn the rotor in a counter clockwise direction by an additional 60 degrees as illustrated in step 2. In Step 3, the switches S3 and S6 are in the on state, so the node 2 is connected to the ground voltage and the node 3 is connected to the positive supply voltage V. As a result the rotor will be rotated by 60 degrees in the counter clockwise direction. The next corresponding three steps (Step1, Step2, Step3) are illustrated in FIG. 3B. By reversing switching patterns of these commutation switches a rotation in a clockwise direction can be achieved.
Therefore, there is a need of a brushless motor drive circuit to provide additional steps in one rotation for a better resolution in each step.